1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a recorded image display apparatus or a facsimile apparatus. Particularly it relates to an image forming apparatus in which the toner density of a two-component developer (developing agent) can be properly controlled.
2. Related Background Art
(a) Cleanerless Process (Toner Recycle Process)
In recent years, the downsizing of image forming apparatuses has progressed, but even if means and instruments for the image forming processes such as charging, exposing, developing, transferring, fixing and cleaning simply become compact, the general downsizing of the image forming apparatuses has been limited. Also, any untransferred toner on a photosensitive member after transfer is collected by cleaning means (cleaner) and becomes a waste toner, but it is preferable from the viewpoint of the protection of environment that this waste toner does not come out. So, there have appeared image forming apparatuses of the "cleanerless process" designed such that the cleaner is detached and the untransferred toner on the photosensitive member is removed and collected from the photosensitive member by "cleaning simultaneous with developing" by developing means and is recycled. The cleaning simultaneous with developing is a method of collecting the untransferred toner somewhat residual on the photosensitive member after transfer by a fog removing bias (a fog removing potential difference Vback which is the potential difference between a DC voltage applied to developing means and the surface potential of the photosensitive member) during the development after the next step. According to this method, the untransferred toner is collected by the developing means and is recycled after the next step and therefore, the waste toner can be eliminated and maintenance can be made unnecessary. Also, being cleanerless leads to a great advantage in terms of space and can greatly downsize the image forming apparatus.
(b) Developer Density Control
In a developing device provided in an image forming apparatus of the electrophotographic type or the electrostatic recording type, use is often made of a two-component developer having toner particles and carrier particles as chief components. Particularly, in color image forming apparatuses for forming full color images or multicolor images by the electro-photographic process, almost all developing devices use the two-component developer from the viewpoint of the color taste of images. As is well known, the toner density (the rate of the weight of toner particles to the total weight of carrier particles and toner particles) of this two-component developer is a very important factor for stabilizing the quality of image. The toner particles of the developer are consumed during development and the toner density is varied. Therefore, it is necessary to accurately detect the toner density of the developer and operate an automatic toner replenishing device (ATR) in conformity with a variation in the toner density to thereby control the toner density so as to be always constant and maintain the quality of images.
Detecting devices and density control devices of various types for detecting the toner density in the developing device in order to correct the density of the developer in the developing device being varied by development as described above, that is, to control the amount of toner replenished to the developing device, have heretofore been proposed and put into practical use. For example, there is a detecting device disposed in proximity to a developer bearing member or the developer carrying path of a developer container, and utilizing that the toner reflects infrared light and that the reflectance thereof differs due to the differences in the spectral sensitivity and light absorptance thereof for each toner density and each color. FIG. 7 shows an example of such a detecting device. The detecting device 10 is provided with a bidirectionally light emitting LED 10a, a light receiving element 10b for reference and a light receiving element 10c for reflection, and one of infrared lights bidirectionally emitted from the LED 10a is applied to the developer carried onto the developer bearing member or the developer in the developer container 1 through transparent acrylic resin 10d, and the amount of reflected light of the infrared light reflected by the developer is received by the light receiving element 10c, and from the light receiving output thereof, the density of the developer is calculated to thereby control toner replenishment (this device will hereinafter be referred to as the "light reflecting type developer density control device").
Or by an LED (light emitting element) 10a similar to that described above, infrared light is applied to a patch image A formed on the image bearing member 1, as shown in FIG. 7, and the amount of reflected light of the infrared light reflected by the patch image is received by the light receiving element 10c, and when from the light receiving output thereof, the patch image A is higher than desired density, toner replenishing is stopped, and when the patch image A is lower than the desired density, toner replenishing is started to thereby indirectly control the density of the developer (this device will hereinafter be referred to as the "patch type developer density control device").
FIG. 2 of the accompanying drawings shows a four-color full color image forming apparatus comprised of a plurality of stations of the aforedescribed cleanerless process type (image forming stations) arranged along a transfer material conveying belt (transfer material conveying means). A toner image formed on the photosensitive drum of each station is sequentially transferred onto a transfer material (e.g. paper) conveyed by the transfer material conveying belt. In the image forming apparatus of such a construction, there may sometimes occur the so-called retransfer which is a phenomenon that when the toner transferred onto the transfer material in the preceding station adheres to the photosensitive drum of the next station when it has arrived at the next station.
Specifically, the full color image forming apparatus is usually provided with four stations Pa, Pb, Pc and Pd of yellow (Y), magenta (M), cyan (C) and black (K), and the transfer of toner images to the transfer material P is effected four times for Y, M, C and K, but during the second, i.e., M, transfer in the station Pb, part of the Y toner already transferred onto the transfer material P may sometimes be returned onto the photosensitive drum 1. Likewise, during the third, i.e., C, transfer in the station Pc, the Y toner and the M toner are retransferred, and during the fourth, i.e., K, transfer in the station Pd, the Y toner, the M toner and the C toner are retransferred. That is, in the M, C and K stations Pb, Pc and Pd, the toners of colors originally different from the toners in those stations are retransferred to the photosensitive drums 1.
The retransfer of the Y toner in the station Pb for forming the M toner will hereinafter be described as an example with reference to typical views. As shown in FIGS. 8A and 8B, usually when there is provided a cleaner 11 for removing any untransferred toner on the photosensitive drum 1, even if the Y toner T on the transfer material P borne on a transfer material conveying belt 5 conveyed in the direction of arrow R5 is retransferred onto the photosensitive drum 1 of the station Pb, it is collected by the cleaning blade 11a of the cleaner 11 and therefore, thereafter it never happens that a different toner is mixed in a developing device 4 through a developing sleeve 4a rotated in the direction of arrow R4. In the case of the cleanerless process, however, the cleaner is absent as shown in FIG. 9A, the Y toner T retransferred to the photosensitive drum 1 as shown in FIG. 9B is collected by the developing device 4 as shown in FIG. 9C. The toner retransferred in this manner may be gradually accumulated in the developing device 4 by image formation being repeated and the color taste of the toner in that developing device 4 may sometimes be changed.
In a system such as the aforedescribed light reflecting type developer density control device or patch type developer density control device wherein developer density control is effected from the optical characteristic of the developer or the toner to the developer of which the color taste has thus been changed, that is, the amount of reflected light relative to light of a certain wavelength has been changed, the amount of reflected light may be varied from the variation in the optical characteristic by the color mixing of the toners and an error may occur to the developer density control in spite of suitable developer density and patch density being actually indicated.
As the result, the image density rise and toner scattering by the excessive replenishing of the toners, or the adherence of the carrier resulting from a reduction in image density and a reduction in developer density by the stoppage of the replenishing of the toners may be sometimes caused.